The operating principles of the laser range finder are divided into the pulse type and the phase type. Similar to the radio detecting and ranging (RADAR), a pulse-type laser range finder transmits a laser signal toward a target, and then the laser signal is reflected back to the laser range finder, whereby the laser finder measures the traveling time (i.e., the fly time) of the laser signal. Accordingly, the distance of the target is derived from the light speed multiplied by half of the fly time. Another, the phase-type range finder emits the laser beam which is modulated continuously to the target and then detects the variance of the phase during the traveling time. By calculating the variance of the phase during the traveling time, the phase-type laser range finder can get the distance easily.
As for the pulse-type laser range finder, the measurement accuracy depends on how to measure the fly time of laser signal. Typically, the pulse-type laser range finder includes an electronic range counter circuit to generate pulse signals at a constant frequency. By counting the pulse signals generated during the fly time, the laser range finder obtains a measured value, which is a positive integer. Thus the frequency at which the pulse signals are generated decides the measurement resolution, and the higher frequency pulse causes the higher resolution.
However, the electronic range counter circuits capable of generating high frequency pulse are costly. Therefore, other approaches to raise the resolution are introduced. For example, U.S. Pat. No. 5,552,878 discloses a vernier for a laser range finder, and more particularly an electronic vernier for increasing the resolution of a laser range finder. The vernier subdivides the clock pulses into a predetermined plurality of equal increments corresponding to respective phases of the clock pulses. Thus the fly time can be measured more accurately without higher frequency pulse. In such design, the measurement accuracy mainly depends on the resolution of the vernier.
Another approaches to raise the resolution relate to the sampling frequency and sampling times. For example, U.S. Pat. No. 6,466,307 discloses a signal processing method of a laser range signal. By setting up measurement block number and the laser emission time and then emitting a laser beam of a predetermined delay time, the accuracy of the laser range finder is enhanced with less memory used. However, this design calls for additional electronic circuits to control sampling frequencies and sampling times and to synchronize multiple electronic signals, so that it raises the cost and complicates the assembling process.
Therefore, it is desirable to have a pulse-type laser ranger finder and a method for a pulse-type laser range finder with high measurement accuracy as the same time when the maximal frequency of the pulse signals is constrained. It will be advantageous if the high measurement accuracy doesn't rely on sampling frequencies and sampling times. Also, it will be more advantageous if the laser finder and the method are able to employ the mechanical design and the manufacture process of the prior art.